Microelectronics packages are required to operate under a variety of conditions, including repetitive or cyclical variations in the temperature of the operating environment. Typically, quality assurance testing of microelectronics packages involves subjecting the packages to repetitive thermal cycling during a procedure known as "burn in" testing. As the trend toward decreasing the size of microelectronic packages continues, the problems associated with repetitive thermal cycling during testing and operation become more pronounced.
FIG. 1 is a partial cross-sectional elevational view of a portion of a micro-ball grid array (micro-BGA) package 10 in accordance with the prior art. The micro-BGA package 10 includes a die 12 having a plurality of bond pads 14 formed thereon. The micro-BGA package 10 also includes an interposer (or lead frame) 16 having a dielectric substrate 18 with a plurality of contact pads 20 formed thereon.
A spacing layer 22 is disposed between the die 12 and the interposer 16, and a plurality of conductive leads 24 coupled between the die 12 to the interposer 16. Each conductive lead 24 has a first end 26 bonded to one of the bond pads 14 and a second end 28 bonded to one of the contact pads 20, thereby electrically coupling the die 12 to the interposer 16. An encapsulating material 30 is disposed over the conductive leads 24 and the exposed areas of the die 12 to seal and protect the fragile conductive leads 24 and circuitry of the die 12 from the environment. Finally, a conductive bump 32 is formed on each of the contact pads 20. Micro-BGA packages of the type shown in FIG. 1, and methods of forming such packages, are shown and described, for example, in U.S. Pat. Nos. 5,663,106 and 5,777,379 to Karavakis et al, and in U.S. Pat. No. 5,821,608 to DiStefano et al, which are incorporated herein by reference.
Typically, the bond pads 14 may be formed of aluminum or other suitable electrically-conductive material while the die 12 is primarily composed of silicon. The dielectric substrate 18 of the interposer 16 may be a molded plastic or ceramic material, and the contact pads 20 may be aluminum or other suitable metallic material. Gold wires are typically used for the conductive leads 24. Due to the significant differences in the coefficient of thermal expansion (CTE) of these materials, significant mechanical stresses may develop in the micro-BGA package 10 due to the CTE mismatch of these components as the package is subjected to a range of temperatures during testing or in operation.
One prominent problem attributable to the differences in CTE of the components of the micro-BGA package is detachment of the first end 26 of the conductive lead 24 from the bond pad 14 of the die 12. Because the interposer 16 and die 12 have different CTE, temperature fluctuations cause mechanical stresses to develop along and within the bond between the first end 26 and the bond pad 14. After repeated thermal cycling, the bond fatigues and the first end 26 of the conductive lead 24 becomes detached from the bond pad 14. The problem of detachment of the conductive lead 24 from the bond pad 14 is commonly referred to as "bond liftoff."
Efforts have been made to prevent bond liftoff of the first end 26 of the conductive lead 24 from the bond pad 14. For example, as described in U.S. Pat. No. 5,821,608, the conductive leads 24 may have a laterally curved or expandable middle section 27 (FIG. 1) that allows the conductive lead 24 to flex and bend slightly during thermal cycling, thereby reducing the mechanical stress on the solder interface. As the micro-BGA package 10 is heated or cooled, the relative movement of the components due to CTE mismatch is taken up by the flexible, bendable middle section 27, preventing stresses from building up in the bond between the bond pad 14 and the first end 26.
Also, the spacing layer 22 may be formed of a complaint or elastomer material that further reduces the stress on the solder interface due to CTE mismatch between the interposer 16 and the die 12, as disclosed in U.S. Pat. Nos. 5,148,265 and 5,148,266, which are incorporated herein by reference. The flexibility of the spacing layer 22 allows relative movement between the die 12 and the interposer 16 during thermal cycling, preventing the development of stresses induced by the CTE mismatch.
To permit the desired flexure of the conductive leads 24 or the spacing layer 22 in the above-described micro-BGA packages 10, the encapsulating material 30 is composed of a material having a low modulus of elasticity, a low bond strength, a high CTE, and a low glass transition temperature. The glass transition temperature (T.sub.G) of a material is the temperature at which an amorphous polymeric material changes from a hard, relatively brittle condition to a soft, relatively rubbery condition. Thus, in the above-described prior art packages, the encapsulating material 30 is typically composed of a soft, compliant polymeric material, such as silicone rubber or other castable elastomer, having a modulus of elasticity typically from about 400 psi to about 800 psi, a CTE from about 100 to about 300 ppm/.degree. C., and T.sub.G from about -120 to about 10.degree. C.
These efforts, however, have not been completely effective in preventing bond liftoff of the conductive leads 24 from the bond pads 14 during repeated thermal cycling or due to other sources of stress.